The present invention relates to a golf ball comprising a liquid center, a rubber thread layer formed around the liquid center and a cover covering the rubber thread layer and more particularly to a golf ball having a liquid center of a large diameter so as to increase flight distance, in which trajectory is optimized by improvement of the ratio of the total surface area of dimples to the total surface area of the golf ball.
Normally, 300 to 500 dimples in number are provided on the surface of a golf ball so as to improve aerodynamic characteristics of the golf ball through increase of the flight distance and optimization of the trajectory.
There are many dimple factors which affect the trajectory of the golf ball. One of the factors which has a large influence on the trajectory is the ratio of the total area of the dimples to the surface area of the golf ball, i.e. surface area occupying rate of the dimples.
Conventionally, in view of the surface area occupying rate of the dimples, there are proposals of the golf ball as listed below.
(1) In U.S. Pat. No. 4,804,189, the surface area occupying rate of the dimples is set to be more than 78% by a combination of two kinds of large and small dimples.
(2) In Japanese Patent Laid-Open Publication No. 3-80876 (U.S. Pat. No. 5,090,705), proposed by the present applicant, the dimples are formed so that a value Y of the surface area occupying rate of the dimples is within the range EQU Y=0.046.times..sup.1/2 -0.172
wherein X represents the total number of dimples.
As shown in FIG. 1, the golf ball G disclosed in the above references has a liquid center 1 and a rubber thread layer 2 formed around the liquid center 1 and a cover 3 covering the rubber thread layer 2, in which a diameter L1 of the liquid center 1 is about 28.3 mm, while an outer diameter L2 of the golf ball G is 42.75 mm.
Recently, in order to increase the flight distance of the golf ball of a liquid center type, a golf ball having a larger diameter owing to increase of the diameter of the liquid center 1 so as not only to decrease amount of spin at the initial stage of flying but also to increase an angle of elevation of trajectory has been dominant. Namely, experiments have revealed that if the diameter of the liquid center of the golf ball is increased, the golf ball becomes softer than the conventional golf ball, thus resulting in increase of amount of deformation of the golf ball and decrease of spin thereof.
However, if the diameter of the liquid center of the golf ball is increased with the conventional surface area occupying rate of the dimples as disclosed in the above references (1) and (2), the surface area occupying rate of dimples becomes too small. As a result, amount of back spin of the golf ball defined by aerodynamic characteristics of the dimples is reduced, so that the lift of the golf ball reduces, and the angle of elevation of trajectory is small, thereby resulting in reduction of carry. Therefore, the golf ball does not have a flight distance longer than the conventional golf ball.
The thread wound golf ball having the liquid center is characterized by its higher controllability by increase of backspin in comparison with a two piece solid golf ball. However, if the surface area occupying rate of the dimples in the thread wound golf ball having the liquid center is small, thereby the amount of back spin reduces and the lift of the golf ball decreases, the thread wound golf ball having the liquid center does not have the advantage of the higher controllability.